A major challenge posed by the exponentially increasing demands for data transfer is the realization of compact EO modulators with wide EO bandwidths, low operating voltage, large extinction, and a small footprint. High-speed and low-power devices are especially important in short-reach networks where the replacement of copper with optical interconnects is sought at shorter and shorter length scales. At the same time, greater component density is also required in longer distance networks.
The distinct requirements for long-haul and short-reach networks have resulted in different material platforms and modulation mechanisms investigated for each. Lithium niobate has been well-established as the standard material used for modulators in long-distance communications for decades. These devices, however, are centimeters long, preventing the use of LiNbO3 in shorter-reach networks that require higher density. Silicon free-carrier modulators have received great interest as candidates for short-reach networks due to the potential for integrating both driver electronics and photonic elements into a single CMOS-compatible platform. It remains a major challenge, however, to achieve simultaneously large bandwidths, low voltages, and large extinction in a single device architecture with a sub-millimeter device length.
Enhancement of optoelectronic device performance for both the LiNbO3 and silicon platforms has been demonstrated utilizing photonic crystal (PC) slow light structures. In LiNbO3, the tunability of the PC band edge has been investigated as a modulation mechanism. A major challenge for these devices is weak overlap between the PC structure and optical mode of the LiNbO3 waveguide, which has since been addressed by using smart cut LiNbO3 films. High speed PC modulators using LiNbO3, however, have not yet been demonstrated. Silicon free-carrier modulators with dispersion-engineered line defect PC waveguides have been demonstrated with a reduced driving voltage and device length resulting from an enhancement in the phase delay proportional to the optical group index. Silicon PC modulators have been reported with 40 Gbps bit rates comparable to those achieved in the highest performing rib waveguide silicon modulators with an order of magnitude smaller footprint, demonstrating the effect of slow light in reducing the required modulator size. Silicon modulators, however, still suffer from low extinction, which limits reach. Compared with the approaches taken thus far on LiNbO3, the dispersion-engineered line defect waveguides on silicon offer the advantage of wideband optical operation for wavelength division multiplexing applications. Dispersion-engineered slot photonic crystal waveguides have been further used to enhance the EO coefficient of EO polymers, showing promise for using such an approach to reduce the voltage of χ(2) modulators.